1. Field of the Invention
This invention relates to endpoint detection in a semiconductor process utilizing a neural network.
2. Background
Today in semiconductor processing, different structures, such as trenches and vias in semiconductor devices are formed by etching processes. FIGS. 1A-1C illustrate the steps in a known process for etching a semiconductor device. As shown in FIG. 1A, a semiconductor device 100 comprises a silicon substrate 102, a polysilicon layer 104, and an insulating layer 106. On insulating layer 106 is deposited a photoresist layer 108. Photoresist layer 108 is patterned into a predetermined structure. The predetermined structure may be, for example, a via hole or trench. Photoresist layer 108 serves as a mask for the etching process.
Then, as shown in FIG. 1B, photoresist layer 108 functions as a mask during the etch process. In the etching process, insulating layer 106 is etched until polysilicon layer 104 is reached. Once polysilicon layer 104 is reached, the etch process is terminated. As shown in FIG. 1C, after the etching process is completed, photoresist layer 108 is removed. The etching process used may be, for example, a wet etching. Alternately, a dry etching, such as ion bombardment or plasma etching, may be preformed.
In semiconductor device manufacturing processes, dry etching is one technique for forming micropatterns. Dry etching is a method of generating plasma in a vacuum using a reactive gas and removing an etching target by using ions, neutral radicals, atoms, and molecules in the plasma. Etching is performed on an etching target until an underlying layer is reached. If etching is continued after the etched target is completely removed, the underlying material may be excessively etched, or the etching shape may be undesirably changed. Therefore, to obtain a desired structure, it is advantageous to detect the endpoint in the etching process accurately.
FIGS. 2A and 2B illustrate etching a semiconductor device by a dry etching process. As shown in FIG. 2A, a semiconductor device 200 comprises a silicon substrate 202, an insulating layer 204, and a photoresist layer 206. Photoresist layer (PR) 206 is patterned into a predetermined structure, such as a trench or via hole.
As shown in FIG. 2B, plasma gas reactant 208 is introduced to semiconductor device 200. Plasma reactant 208 reacts with layer 204, thereby etching layer 204. As plasma reactant 208 reacts with layer 204, a by-product is produced, the significance of which will be described hereinafter. Once layer 204 is etched so that substrate 202 is exposed, plasma reactant 208 is removed, thereby ending the etching process.
The point at which the etching process is stopped is called the endpoint. There are several methods in determining the endpoint in an etching process. Optical emission spectroscopy is one method for endpoint detection. Optical emission spectroscopy is easy to implement and offers high sensitivity. Optical emission spectroscopy relies on the change in the emission intensity of characteristic optical radiation from either a reactant or by-product in a plasma. Light is emitted by excited atoms or molecules when electrons relax from a higher energy state to a lower one. Atoms and molecules emit a series of spectral lines that are unique to each individual reactant or by-product. FIG. 3 is a graph illustrating the emission spectrum of exemplary by-products in a conventional etching process. As seen in FIG. 3, each different reaction product emits radiation at a different wavelength. The emission intensity is a function of the relative concentration of a species in the plasma. By observing the intensity of a particular by-product, the endpoint of the etch process may be determined.
A known system 400 utilized for endpoint detection is shown in FIG. 4. System 400 comprises a reaction chamber 410 which contains a semiconductor device 412 to be processed and a window 414. System 400 operates by recording with a recorder 440, such as a meter or oscilloscope, the emission spectrum, otherwise known as an endpoint curve, during the etch process in the presence and absence of the material that is to be etched. Emission radiation from the reaction passes through window 414 through a lens 420 and is received by a detector 430. Detector 430 is equipped with a filter that lets light of specific wavelengths pass through for detection. To detect the endpoint, the emission intensity of a particular reactant or by-product is monitored at a fixed wavelength and recorded on a recorder 440. When the end point is reached, the emission intensity changes. The change in emission intensity at the endpoint depends on individual reactant or by-product being monitored. The intensity due to reactant increases, while the intensity due to etch by-products decreases as the endpoint is reached.
FIG. 5 is a graph illustrating the emission spectrum intensity for a by-product in an etch process. As shown in the graph, as the etch process begins (502), the intensity of the emissions for the by-product from the reaction process increase as a function of the elapsed time (504). Then, as the etch process nears the endpoint (506), the emission intensity for the by-product decreases. Thus, the endpoint (506) can be determined by monitoring the emission of a reaction product for a decrease in intensity. FIG. 6 is a graph illustrating an experimental endpoint curve for a exemplary by-product. The endpoint is determined from observing the curve to locate normal features that denote an endpoint. Normal features are features which regularly occur in an endpoint curve.
Optical emissions spectroscopy also has some drawbacks. Oftentimes, it is difficult to determine the exact endpoint of an etching process. Such is the case when an endpoint curve contains abnormal features that may be hard to detect by conventional optical spectroscopy. Many times an over-etching is required. However, in the case of overetching, the underlying materials may be damaged.